(a) Field of Invention
The present disclosure of invention relates to methods for driving pixel areas of an electrophoretic display.
(b) Description of Related Art
Recently, for replacing the conventional CRT tubes; flat panel displays, such as liquid crystal displays, organic light emitting diode displays, electrophoretic displays, and so on, have been developed.
Among the above flat panel displays, the typical electrophoretic display includes: (1) a thin film transistors (TFT) array supporting panel including pixel electrodes each connected to a thin film field effect transistor; (2) a common electrode supporting panel having a common electrode included thereon, and (3) a layer of electrophoretic particles that have positive or negative charges which are positioned in respective pixel regions, and move between the pixel electrodes of the respective pixel areas and the common electrode. A combination of a single pixel electrode and its opposed area in the spaced-away common electrode and its TFT and the eletrophoretic particles disposed between the pixel electrode and common electrode may be considered as a pixel unit that can be driven by electromotive forces at least between first and second different optical states (i.e. red and black).
When different data voltages are applied to the pixel electrodes relative to and a common voltage applied to the common electrode, the differences between the data voltages and the common voltage can be sufficient to generate electromotive forces that rearrange the electrophoretic particles that are disposed in the respective pixel regions so as to provide a desired optical state. A first polarity set of the electrophoretic particles having positive or negative charges are attracted to move adjacent to the pixel electrodes while the opposed, second polarity set of particles are attracted to move adjacent to the common electrode by use of a first polarity of driving voltage. Along with the voltage-mediated re-arrangement of the electrophoretic particles, an external light applied to the electrophoretic display may be absorbed by or reflected by the electrophoretic particles, to thereby display the corresponding pixel area as being of a respective black or white or other colored attribute to a user who is looking at the display.
While some parts of a displayed image may be constantly changing between opposed states (e.g., first displaying white and then black), it is often the case that other parts of the displayed image persistently remain in a same state (e.g., displaying just black as a background color for example) for prolonged periods of time (e.g., 3 seconds or more). In these relatively unchanging areas, it is conventional to apply the same positive or negative driving voltage constantly to the electrophoretic particles for the duration of the time that the user is intended to perceive the area as having a constant black or white or other color. However, when a data voltage having the same polarity and magnitude is re-applied periodically to the pixel electrode of the corresponding pixel region through the thin film transistor for a long period of time, the electrical current that flows through the corresponding thin film transistor (TFT) in order to charge the corresponding pixel-electrode to the desired constant voltage flows in only one direction. (The capacitance of the pixel unit discharges internally after a while and needs to be refreshed in order to maintain the desired persistent optical state.) When current flows through some thin film transistors in only one direction, a current-induced deterioration of the thin film transistor (i.e., due to electromigration) is accelerated as compared to the case in which current flows alternately in opposite directions through a TFT of same structure.